This invention relates generally to fiber optics and more specifically to an apparatus for reducing the polarization mode dispersion in a fiber optic transmission system. Polarization mode dispersion (PMD) refers to variations in the time delay of a polarized optical signal traveling through an optical transmission system, such as a single-mode optical fiber. PMD arises in an optical fiber as a result of asymmetries in the optical fiber's core, such as core ellipticity created during optical manufacturing and bending stresses formed during installation. The asymmetries of the fiber's core cause random changes in the state of polarization (SOP) of optical signals propagating through the fiber. Different SOPs propagate through the optical fiber core at different relative speeds, e.g., some SOPs travel faster and some travel slower, resulting in a pulse width distortion of a transmitted optical signal relative to an input optical signal. Additionally, the asymmetries of the fiber's core are highly susceptible to environmental fluctuations, such as temperature or movement of the fiber, which occur as fast as milliseconds and result in a time varying pulse width distortion of the transmitted optical signal.
The varying pulse width distortion is mathematically represented by a time delay between two orthogonally polarized principal states of polarization (“PSP”) which form a convenient basis set to describe and characterize each SOP, and evaluate the effects of PMD in the fiber. Using the PSPs as a basis set, each SOP propagating through an optical fiber is represented by a linear combination of the two orthogonally polarized PSPs. The varying pulse width distortion of the SOP is a function of a varying delay between the PSPs. Theoretically, each PSP experiences a time of flight difference through the optical fiber, commonly known as differential group delay, resulting in a time delay between the two PSPs at the fiber output. The output SOP is represented by a linear combination of the PSPs which are time delayed with respect to each other. A greater time delay between the PSPs corresponds to a larger relative difference between the input SOP pulse width and the output SOP pulse width. See for example C. D. Poole and R. E. Wagner, “A Phenomenological Approach to Polarization Dispersion in Long Single-Mode Fibers.” Electronic Letters, Vol. 22, pp. 1029-1030, September 1986, which is incorporated by reference herein.
Optical fibers have a differential group delay (DGD) between the two PSPs on the order of 0.1 ps/km. In older fiber optic cables, such as the cables used in terrestrial networks, the DGD is on the order of 2.0 ps/km and results in time delays of about 50 picoseconds for transmission distances of only several hundred kilometers. As the demand for faster optical data transmission increases, such as from gigabits per second to terabits per second, optical pulse width distortion due to PMD will become one of the factors limiting data transmission rate.